Roslyn compilation doesn't resolve mscorlib references

Question

I'm working on a project where I compile projects from a solution using Roslyn.

foreach (var projectId in solution.GetProjectDependencyGraph().GetTopologicallySortedProjects())
{
    var project = solution.GetProject(projectId);
    var compilation = project.GetCompilationAsync().Result;
    var errors = compilation.GetDiagnostics().Where(d => d.Severity == DiagnosticSeverity.Error);
    // ...

Compilation contains errors such as

error CS0012: The type 'Task' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Threading.Tasks, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'.

Why Roslyn won't accept an existing reference to mscorlib?

• msdn for System.Threading.Task points out that this type is indeed located in mscorlib.dll
• PublicKeyToken b03f5f7f11d50a3a matches mscorlib's

Are there some CompilationOptions that I should consider? Per this thread I tried assemblyIdentityComparer: DesktopAssemblyIdentityComparer.Default but it didn't help. I tried to work with metadataReferenceResolver but couldn't find much information about it.

Following the solution in Roslyn has no reference to System.Runtime I implemented code that ensures that a project has references to mscorlib.dll, System.Core.dll, System.dll and System.Runtime.dll, such that my project and compilation have the references:

Side note: Reference #7 has been added this way. The project already had references #1, 2 and 3, and removing them and replacing with ones from C:\Windows\Microsoft.NET\Framework didn't solve the issue.

project.MetadataReferences.ToList()
Count = 8
    [0]: Assembly Path='C:\Program Files (x86)\Microsoft Visual Studio 14.0\Common7\IDE\PublicAssemblies\Microsoft.VisualStudio.QualityTools.UnitTestFramework.dll'
    [1]: Assembly Path='C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.5\mscorlib.dll'
    [2]: Assembly Path='C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.5\System.Core.dll'
    [3]: Assembly Path='C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.5\System.dll'
    [4]: Assembly Path='C:\Users\Amadeus\Documents\GitHub\InterProcessQueue\src\MemoryMappedQueue\packages\xunit.runner.visualstudio.2.0.0-rc1-build1030\build\_common\xunit.abstractions.dll'
    [5]: Assembly Path='C:\Users\Amadeus\Documents\GitHub\InterProcessQueue\src\MemoryMappedQueue\packages\xunit.assert.2.0.0-rc1-build2826\lib\portable-net45+aspnetcore50+win+wpa81+wp80+monotouch+monoandroid+Xamarin.iOS\xunit.assert.dll'
    [6]: Assembly Path='C:\Users\Amadeus\Documents\GitHub\InterProcessQueue\src\MemoryMappedQueue\packages\xunit.extensibility.core.2.0.0-rc1-build2826\lib\portable-net45+aspnetcore50+win+wpa81+wp80+monotouch+monoandroid+Xamarin.iOS\xunit.core.dll'
    [7]: Assembly Path='C:\Windows\Microsoft.NET\Framework\v4.0.30319\System.Runtime.dll'

compilation.ExternalReferences.ToList()
Count = 9
    [0]: Assembly Path='C:\Program Files (x86)\Microsoft Visual Studio 14.0\Common7\IDE\PublicAssemblies\Microsoft.VisualStudio.QualityTools.UnitTestFramework.dll'
    [1]: Assembly Path='C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.5\mscorlib.dll'
    [2]: Assembly Path='C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.5\System.Core.dll'
    [3]: Assembly Path='C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.5\System.dll'
    [4]: Assembly Path='C:\Users\Amadeus\Documents\GitHub\InterProcessQueue\src\MemoryMappedQueue\packages\xunit.runner.visualstudio.2.0.0-rc1-build1030\build\_common\xunit.abstractions.dll'
    [5]: Assembly Path='C:\Users\Amadeus\Documents\GitHub\InterProcessQueue\src\MemoryMappedQueue\packages\xunit.assert.2.0.0-rc1-build2826\lib\portable-net45+aspnetcore50+win+wpa81+wp80+monotouch+monoandroid+Xamarin.iOS\xunit.assert.dll'
    [6]: Assembly Path='C:\Users\Amadeus\Documents\GitHub\InterProcessQueue\src\MemoryMappedQueue\packages\xunit.extensibility.core.2.0.0-rc1-build2826\lib\portable-net45+aspnetcore50+win+wpa81+wp80+monotouch+monoandroid+Xamarin.iOS\xunit.core.dll'
    [7]: Assembly Path='C:\Windows\Microsoft.NET\Framework\v4.0.30319\System.Runtime.dll'
    [8]: Compilation (C#): MemoryMappedQueue

• How can I get Roslyn to compile this project?
• Are there any CompilationOptions I should use?
• Is Roslyn's issue #970 related to this?

Answer 1

As per Kevin's answer, this can be resolved by specifying a property for MSBuildWorkspace:

var props = new Dictionary<string, string>();
props["CheckForSystemRuntimeDependency"] = "true";
var msWorkspace = MSBuildWorkspace.Create(props);

Now solutions opened in msWorkspace will correctly resolve their references.